Marine mammals cannot sleep underwater because they need to surface occasionally to take a breath. It is also impossible to remain on the surface – due to the danger of attack by predators and heat loss. How do they get out? Live Science learned from biologists.

Nature’s most ingenious invention to solve this dilemma is unihemispheric sleep. It is characteristic of cetaceans, and the most famous example is dolphins.

“Single-hemispheric sleep is really valuable for these animals because it allows them to maintain a low level of activity while only half the brain sleeps at a time,” explains the biologist at the University of St. Andrews in the UK. Patrick Miller.

This sleeping style is not exclusive to the inhabitants of the deep sea. It is practiced by some birds, although for other purposes – they doze with half their eyes when flying in a flock.

On the other hand, not all cetaceans are capable of unihemispheric sleep. Some of them sleep, like people and other animals – with both hemispheres. Whales, for example, dive into the water, fall asleep and slowly rise to the surface thanks to spermaceti, which gives them buoyancy. However, it is impossible to reliably judge their sleeping habits, Miller complains.

“It is very difficult to measure the brain activity of marine animals that cannot be caught, such as sperm whales, blue whales or humpback whales. In this case, behavioral records are the best indicator of sleep behavior,” he shares.

Miller and his colleagues sculpted special marks on sperm whales. It turned out that entire schools of whales hung in the water column with their noses up for 20 minutes (apparently, a deep sleep phase), surfaced to catch their breath, and sank again. And so on – up to 3.5 hours.

Judging by observations of elephant seals, which sleep the same way, they are paralyzed during REM sleep, just like you and me. This discovery was made by a postdoctoral fellow at the Scripps Institution of Oceanography at the University of California, San Diego. Jessica Kendall-Bar, who shared her 2023 study on YouTube. She noticed that the seals dive to a depth of about 300 meters, turn upside down and spin slowly. At this point they are probably dreaming.

To avoid becoming prey to predators, elephant seals sleep very little – up to two hours a day. In this they can compete with the African elephant, the record holder among mammals for the minimum amount of sleep, Kendall-Bar noted.